It's possible to add tracks in the Cue window the usual way, either by double-clicking on the free space in the canvas or by choosing the Track > Add Track, Bus, or VCA menu item. The important part is to enable the Show on Cue Page checkbox in the newly opened dialog.
Alternatively, it's possible to drop an existing audio or MIDI file into the empty part of the canvas to automatically create the cue track and fill the first empty trigger slot with it.
Trigger slots not only work as a drag-and-drop target, they also work as a drag-and-drop sources. It's possible to drag the contents of an existing trigger slot into the empty part of the canvas to create a new track and fill the first slot with the contents of the original slot. It's also possible to drag and drop the contents of one slot into another in case you want to use the same clip with some variations (e.g. a custom follow length).
There are several ways to load a clip into a trigger slot:
The type of data must match the type of tracks a clip is being inserted to. E.g. it's impossible to place a MIDI clip into a trigger slot of an audio track.